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Random Quote: They talk of my drinking, but never of my thirst.
After a few hours of running my electronics (Furuno 1823C) with the engine off the battery voltage drops enough so that when I start the engine the unit reboots.
Is there any type of marine grade capacitor or device available that will prevent this type of sudden drop in voltage? Thanks, Wayne.
I can imagine a pretty simple circuit with a couple of diodes and a small gel cell battery that may work. BUT, why not a seperate "house" deep cycle battery for the boat? Use a newmar isolator to charge it.
quote:Originally posted by diver dave:
I can imagine a pretty simple circuit with a couple of diodes and a small gel cell battery that may work. BUT, why not a seperate "house" deep cycle battery for the boat? Use a newmar isolator to charge it.
I have two batteries, but keep one in reserve. My batteries are located in the console and space is tight, so I'd prefer not to have a third batterie.
If you want to invest $10 in Radio Shack parts, I can suggest a circuit that should fix the problem.
1-4700 microfarad capacitor @ 35 volts. (272-1022).
1-Rectifier Diode 6A @ 50 PIV. They come in a pack of 4. (276-1661)
1-10 ohm resistor. Pack of 2. (271-132)
Connect the resistor in parallel with the diode by twisting the leads together. Connect this assembly to the plus side of the capacitor as follows: The anode or plus side of the diode should connect to the capacitor plus. Connect the minus side of the capacitor to ground. Connect the other side of the diode/resistor assembly to the load side of the fuse that feeds the Furuno.
The capacitor charges slowly through the 10 ohm resistor. When you start your engine and the supply voltage drops, the capacitor voltage feeds the Furuno through the diode and hopefully keeps it running.
I had a LMS 350A that had the same problem and the above circuit worked fine.
Birdy;
Do you really have a similar problem or is it just a nice-to-have thing? Wayne only needed a clamp (that's what they call those circuits) for one piece of equipment and the electical resistance of the fuse to his Furuno is one of the components that make the clamp work. I would not have been surprised if Wayne had come back and said "it blew the fuse", and I was fully prepared to tell him to put in a slo-blo fuse. Anyway, the fuse resistance isolates (here we are back on isolators again) the capacitor voltage from the rapidly dropping 12 volt supply and, via the diode, holds the Furuno input voltage high enough to prevent shutdown.
I do have another circuit that should work OK for your application, ie, all of your electronics, but your electronics will have to operate on a 12 volt supply that is a half volt less than battery output voltage. If you think that is not a problem for you, let me know. Only real difference in components is yours requires 2 higher rated diodes and a slightly different circuit configuration.
I'm outa here, be back Monday.
After thinking about this this morning, really, the only peiece of equipment I have a problem with (every once in a while) is the chartplotter, cuase when it restarts, ya have to re-set up some settings (head up, turn radar on, correct scale...). The other stuff really isn't affected in any way....
I think I'll try just what you have above. Only thing is, I don't have a fuse on the chartplotter, just a breaker. I would assume that would have about the same resistance as the fuse, yes?
Under normal power level of the battery, the capacitor is charged up by way of the resistor.
While cranking as the battery voltage drops, the capacitor supplies voltage by way of the diode.
However there is something that than can make this disfunctional.
It doesnt actually depends on the fuses resistance, which is negible by the way, it depends on the resistance of the wire coming from the battery to the fuse to the load, and the return wire back to the battery.
So if the load doesnt draw much power I.E. low current flow, there will be little if any voltage drop across the wires coming from the battery. And thus no isolation from the starter will be seen. The cap will not be supply enough current to power the load and the starter, and the loads voltage will get dragged down just as though the cap wasnt even there.
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A better way to do this is to use a slow blow fuse, drop the resistor, wire the cap directly across the load, and replace the diode with a schottkey power diode, www.digikey.com has them. Schottkeys drop very little voltage at rated current.
Wire the diode with the anode connected to the fuse, and the cathode connected to the load/capacitor.
This way when the key is turned on the diode will power the load, and charge the cap. As you crank the motor the diode will be reverse biased and will allow the cap to supply the load without being discharged by the starter.
A response mentioned that the newmar unit did not operate up to expectations. I also remember the cap /diode/resistor approach.
I asked a EE at work and we layed out a design with a 2 diodes and a 12 v 4AH battery.
I have the room to install a battery of that size but others might not. The only instrument I'm trying to power is my Chartplotter. I plan to install/test this off season.
Another solution would be to wire your electronics off of the "house" battery.
landlubber, in my (visionary circuit ) the diode acts as a switch to remove the load and the cap from the battery while starting.
When the voltage on the cathode of a diode becomes higher than the anode voltage the diode is considered "reversed biased". It will not conduct current other than reverse leakage current which is in the millionths to thousandths of an amp.
By the way I am an EE. I am not trying to take away from the original approach I think its clever. But I just wanted to point out a mode of failure it could experience, and offer possibly a more robust method to achieve the same result.
quote:Originally posted by landlubber:
put battery switch to both/all when starting?
NOT!!
Do that, and run the risk of draining your last good (charged) battery into your other battery which of course is bad (physically, like a bad cell..). Now you can't start the boat at all and the least of yoru problems is weather you have to set up your electronics again from shutting down temporarily.
Besides, who wants to walk to the battery switchs, change em, start the boat, then change em back again.... just to move the boat 50 yards to make another drift? The whole point here is to save time and effort when fishing and having to re-start the motors often to move when drifting.....
Sea_Dad,
Got part numbers? Lets try to keep this in NON_EE talk please! How about explaining it more like this: "Connect the green wire to the little funny looking pinb in the middle of the black thing with three pins on it...."
Sure Birdman, Tell me what your current requirements are, I'll suggest the part numbers, and see if I cant find a way to describe the wiring or find a way to draw a schematic for you.
Maintain Voltage Drop Capacitors ??? 
How about explaining it more like this: "Connect the green wire to the little funny looking pinb in the middle of the black thing with three pins on it...." 
